Soybean Planting Rate and Row Spacing (5/19/11)

In 2011, soybean will continue to be the most common broadleaf field crop grown in North Dakota. Farmers are interested in the combination of production management strategies to economically improve soybean yields. North Dakota State University is conducting several research trials to assist with this goal.

A soybean intensive management study has been conducted since 2008 to examine combinations of planting rates, row spacing, and special foliar inputs using early and late-maturing varieties to identify the most profitable combination. Best management practices are used in the study including seed inoculation, and seed treatment with a fungicide and insecticide. Six site-years of data have been generated from trial locations at Carrington and Prosper. The research is being supported by the North Dakota Soybean Council.

Planting rates of 150,000 and 200,000 pure live seeds (PLS) per acre have been compared, resulting in an average established stand of 138, 000 and 175,000 plants per acre, respectively. NDSU currently recommends an established soybean stand of 150,000 plants per acre, with variance of 10 percent, to maximize yield potential. Results from the study indicates a yield advantage of just under 1 bushel per acre or a yield increase of 1.5 percent averaged across site-years for the high planting rate. However, when costs and benefits are calculated the lower planting rate is more economical.

Fourteen-inch row spacing has averaged 1.1 bushels per acre or about 2 percent greater yield than using 28-inch rows. This confirms other university data indicating a higher yield potential with intermediate rows versus wide rows. Also in the NDSU study, canopy closure occurred an average of about a month earlier with the 14-inch compared to wider 28-inch rows. Quicker canopy closure provides advantages including greater weed competition, soil moisture conservation, and increased capture of sunlight, all potentially resulting in higher yield using narrower rows.

Special foliar inputs, including a nutrient combination plus a growth promoter were applied at the early vegetative stage. This was followed by a fungicide application during the flowering to early-pod formation stages. Across site-years, special inputs increased soybean yield by 2.2 bushels per acre or about 4 percent, compared with the untreated check. However, there was only a modest return-on-investment. The study results indicate the combination of planting 150,000 PLS/acre in 14-inch rows followed by the combination of special foliar inputs provide the highest return on investment among options explored in the study.

In another on-going study conducted at Carrington to examine special inputs for soybean, numerous individual products or combinations applied at various plant stages have not consistently provided yield gain or economic returns. Producers should use caution when considering additional inputs beyond management practices recommended based on university research.